﻿// Copyright (c) Microsoft. All rights reserved.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Windows.Storage;
using Windows.Storage.Streams;
using System.Runtime.Serialization;
using System.IO;
using Windows.ApplicationModel;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;

/// <summary>
/// SuspensionManager captures global session state to simplify process lifetime management
/// for an application.  Note that session state will be automatically cleared under a variety
/// of conditions and should only be used to store information that would be convenient to
/// carry across sessions, but that should be discarded when an application crashes or is
/// upgraded.
/// </summary>
class SuspensionManager
{
    private static Dictionary<string, object> _sessionState = new Dictionary<string, object>();
    private static List<Type> _knownTypes = new List<Type>();
    private const string sessionStateFilename = "_sessionState.xml";

    /// <summary>
    /// Provides access to global session state for the current session.  This state is
    /// serialized by <see cref="SaveAsync"/> and restored by
    /// <see cref="RestoreAsync"/>, so values must be serializable by
    /// <see cref="DataContractSerializer"/> and should be as compact as possible.  Strings
    /// and other self-contained data types are strongly recommended.
    /// </summary>
    public static Dictionary<string, object> SessionState
    {
        get { return _sessionState; }
    }

    /// <summary>
    /// List of custom types provided to the <see cref="DataContractSerializer"/> when
    /// reading and writing session state.  Initially empty, additional types may be
    /// added to customize the serialization process.
    /// </summary>
    public static List<Type> KnownTypes
    {
        get { return _knownTypes; }
    }

    /// <summary>
    /// Save the current <see cref="SessionState"/>.  Any <see cref="Frame"/> instances
    /// registered with <see cref="RegisterFrame"/> will also preserve their current
    /// navigation stack, which in turn gives their active <see cref="Page"/> an opportunity
    /// to save its state.
    /// </summary>
    /// <returns>An asynchronous task that reflects when session state has been saved.</returns>
    public static async Task SaveAsync()
    {
        try
        {
            // Save the navigation state for all registered frames
            foreach (var weakFrameReference in _registeredFrames)
            {
                Frame frame;
                if (weakFrameReference.TryGetTarget(out frame))
                {
                    SaveFrameNavigationState(frame);
                }
            }

            // Serialize the session state synchronously to avoid asynchronous access to shared
            // state
            MemoryStream sessionData = new MemoryStream();
            DataContractSerializer serializer = new DataContractSerializer(typeof(Dictionary<string, object>), _knownTypes);
            serializer.WriteObject(sessionData, _sessionState);
        
            // Get an output stream for the SessionState file and write the state asynchronously
            StorageFile file = await ApplicationData.Current.LocalFolder.CreateFileAsync(sessionStateFilename, CreationCollisionOption.ReplaceExisting);
            using (Stream fileStream = await file.OpenStreamForWriteAsync())
            {
                sessionData.Seek(0, SeekOrigin.Begin);
                await sessionData.CopyToAsync(fileStream);
            }
        }
        catch (Exception e)
        {
            throw new SuspensionManagerException(e);
        }
    }

    /// <summary>
    /// Restores previously saved <see cref="SessionState"/>.  Any <see cref="Frame"/> instances
    /// registered with <see cref="RegisterFrame"/> will also restore their prior navigation
    /// state, which in turn gives their active <see cref="Page"/> an opportunity restore its
    /// state.
    /// </summary>
    /// <returns>An asynchronous task that reflects when session state has been read.  The
    /// content of <see cref="SessionState"/> should not be relied upon until this task
    /// completes.</returns>
    public static async Task RestoreAsync()
    {
        _sessionState = new Dictionary<String, Object>();

        try
        {
            // Get the input stream for the SessionState file
            StorageFile file = await ApplicationData.Current.LocalFolder.GetFileAsync(sessionStateFilename);
            using (IInputStream inStream = await file.OpenSequentialReadAsync())
            {
                // Deserialize the Session State
                DataContractSerializer serializer = new DataContractSerializer(typeof(Dictionary<string, object>), _knownTypes);
                _sessionState = (Dictionary<string, object>)serializer.ReadObject(inStream.AsStreamForRead());
            }

            // Restore any registered frames to their saved state
            foreach (var weakFrameReference in _registeredFrames)
            {
                Frame frame;
                if (weakFrameReference.TryGetTarget(out frame))
                {
                    frame.ClearValue(FrameSessionStateProperty);
                    RestoreFrameNavigationState(frame);
                }
            }
        }
        catch (Exception e)
        {
            throw new SuspensionManagerException(e);
        }
    }

    private static DependencyProperty FrameSessionStateKeyProperty =
        DependencyProperty.RegisterAttached("_FrameSessionStateKey", typeof(String), typeof(SuspensionManager), null);
    private static DependencyProperty FrameSessionStateProperty =
        DependencyProperty.RegisterAttached("_FrameSessionState", typeof(Dictionary<String, Object>), typeof(SuspensionManager), null);
    private static List<WeakReference<Frame>> _registeredFrames = new List<WeakReference<Frame>>();

    /// <summary>
    /// Registers a <see cref="Frame"/> instance to allow its navigation history to be saved to
    /// and restored from <see cref="SessionState"/>.  Frames should be registered once
    /// immediately after creation if they will participate in session state management.  Upon
    /// registration if state has already been restored for the specified key
    /// the navigation history will immediately be restored.  Subsequent invocations of
    /// <see cref="RestoreAsync"/> will also restore navigation history.
    /// </summary>
    /// <param name="frame">An instance whose navigation history should be managed by
    /// <see cref="SuspensionManager"/></param>
    /// <param name="sessionStateKey">A unique key into <see cref="SessionState"/> used to
    /// store navigation-related information.</param>
    public static void RegisterFrame(Frame frame, String sessionStateKey)
    {
        if (frame.GetValue(FrameSessionStateKeyProperty) != null)
        {
            throw new InvalidOperationException("Frames can only be registered to one session state key");
        }

        if (frame.GetValue(FrameSessionStateProperty) != null)
        {
            throw new InvalidOperationException("Frames must be either be registered before accessing frame session state, or not registered at all");
        }

        // Use a dependency property to associate the session key with a frame, and keep a list of frames whose
        // navigation state should be managed
        frame.SetValue(FrameSessionStateKeyProperty, sessionStateKey);
        _registeredFrames.Add(new WeakReference<Frame>(frame));

        // Check to see if navigation state can be restored
        RestoreFrameNavigationState(frame);
    }

    /// <summary>
    /// Disassociates a <see cref="Frame"/> previously registered by <see cref="RegisterFrame"/>
    /// from <see cref="SessionState"/>.  Any navigation state previously captured will be
    /// removed.
    /// </summary>
    /// <param name="frame">An instance whose navigation history should no longer be
    /// managed.</param>
    public static void UnregisterFrame(Frame frame)
    {
        // Remove session state and remove the frame from the list of frames whose navigation
        // state will be saved (along with any weak references that are no longer reachable)
        SessionState.Remove((String)frame.GetValue(FrameSessionStateKeyProperty));
        _registeredFrames.RemoveAll((weakFrameReference) =>
        {
            Frame testFrame;
            return !weakFrameReference.TryGetTarget(out testFrame) || testFrame == frame;
        });
    }

    /// <summary>
    /// Provides storage for session state associated with the specified <see cref="Frame"/>.
    /// Frames that have been previously registered with <see cref="RegisterFrame"/> have
    /// their session state saved and restored automatically as a part of the global
    /// <see cref="SessionState"/>.  Frames that are not registered have transient state
    /// that can still be useful when restoring pages that have been discarded from the
    /// navigation cache.
    /// </summary>
    /// <remarks>Apps may choose to rely on <see cref="LayoutAwarePage"/> to manage
    /// page-specific state instead of working with frame session state directly.</remarks>
    /// <param name="frame">The instance for which session state is desired.</param>
    /// <returns>A collection of state subject to the same serialization mechanism as
    /// <see cref="SessionState"/>.</returns>
    public static Dictionary<String, Object> SessionStateForFrame(Frame frame)
    {
        var frameState = (Dictionary<String, Object>)frame.GetValue(FrameSessionStateProperty);

        if (frameState == null)
        {
            var frameSessionKey = (String)frame.GetValue(FrameSessionStateKeyProperty);
            if (frameSessionKey != null)
            {
                // Registered frames reflect the corresponding session state
                if (!_sessionState.ContainsKey(frameSessionKey))
                {
                    _sessionState[frameSessionKey] = new Dictionary<String, Object>();
                }
                frameState = (Dictionary<String, Object>)_sessionState[frameSessionKey];
            }
            else
            {
                // Frames that aren't registered have transient state
                frameState = new Dictionary<String, Object>();
            }
            frame.SetValue(FrameSessionStateProperty, frameState);
        }
        return frameState;
    }

    private static void RestoreFrameNavigationState(Frame frame)
    {
        var frameState = SessionStateForFrame(frame);
        if (frameState.ContainsKey("Navigation"))
        {
            frame.SetNavigationState((String)frameState["Navigation"]);
        }
    }

    private static void SaveFrameNavigationState(Frame frame)
    {
        var frameState = SessionStateForFrame(frame);
        frameState["Navigation"] = frame.GetNavigationState();
    }
}

public class SuspensionManagerException : Exception
{
    public SuspensionManagerException()
    {
    }

    public SuspensionManagerException(Exception e) : base("SuspensionManager failed", e)
    {
        
    }
}
